X-ray Joins The Digital Age
Using X-ray inspection for fault analysis and process control has been used by the semiconductor and electronics industry for some time. The cost and difficulty in use has meant it has not been as widely used as possible. In the past, X-ray inspection was only available to those who could not only afford it, but also had the skill and knowledge to operate the equipment and interpret the resulting images. For example, even under expert control the typically complicated, joystick-based, system controls could make it difficult to prevent a sample from colliding with, and being damaged by, other parts of the X-ray system.
What often made this worse was that the X-ray images were produced on film and so required wet chemistry expertise to develop the images before even attempting to diagnose the fault. Film has generally been replaced by real-time X-ray imaging techniques images are available very quickly. However, producing useful X-ray images is still seen by many as the domain of the expert.
Dage Precision Industries have been manufacturers of X-ray test systems for 3 years since the purchase of an X-Ray company. They have utilised their twenty five years of experience in inspection tools and have taken the X-ray process to the next logical step by integrating digital image acquisition technology (XiDAT) as a replacement for analogue x-ray imaging. The digital data processing allows the system to generate an enhanced resolution and extensive greyscale definition for current and future industry needs.
X-head Analogue to Digital
Analogue and digital systems have a similarity made up from the same basic parts:
* Analogue to Digital converter (only necessary on analogue systems)
* Frame Grabber
* Image processing computer
Digital detection techniques have provided enhancements throughout the inspection process. In particular; digital transmission of video signals can give a significant improvement in quality because of less noise, lower conversion losses and an absence of analogue signal degradation. Additionally, the technology in high-grade digital sensors continues to expand whilst the associated cost falls. With this in mind it is now possible to utilise the latest techniques within the X-ray inspection arena.
A close working relationship with sensor manufacturers has been an important part of the process required to develop the new tool. Utilising sophisticated data handling software it has been possible to eliminate the disadvantages previously associated with integrating image signal within the sensor itself. The previous method meant poor quality live images and the need to back up a digital detector with a conventional image intensifier system. The new digital approach eliminates the need for these additives. The latest sensors, combined with the correct software, are now able to deliver a constant live image as well as allowing all of the standard data processing functions that operators require.
Acquiring images with a resolution of 1.3 million pixels and over 65,000 levels of greyscale represents a significant improvement over the existing analogue technology and also provides over four times the number of pixels compared to analogue systems. This ability allows the system to identify and analyse much larger images with increased levels of detail, resulting in increased inspection throughput. Additionally, this resolution is available on live images at 25 frames per second and image enhancement can be achieved using a full range of digital picture processing techniques.
The imaging chain of the XiDAT technique can resolve four times the number of greyscale levels compared to analogue systems allowing the detection of small differences in the level of grey, such as those found in low contrast samples (e.g. in die attach voiding and power devices). This is achieved by passing the digital information from the point of imaging directly to the host PC, which passes it directly in digital form to the high resolution monitor, eliminating all unnecessary analogue signal paths. By utilising this technology, images are viewed in real time with zero conversion loss and zero loss of image resolution.
Improving Inspection Throughput
Many companies have been ruled out using X-ray inspection as a routine quality control tool within the production process due to labour and resource limitations. Unfortunately, these decisions come at a time when the industry strives to reduce the physical size of the components and increase the relative complexity. With the connections of these devices unable to be seen optically in a routine manner, the availability and use of X-rays becomes the only way to reliably confirm the quality of production. However, just being able to see the joints is not enough. In order to be competitive, X-ray manufacturers and their customers must provide fault information faster and be able to inspect more samples in less time.
Apart from making the actual physical operation of the inspection system far more simple and intuitive, X-ray equipment manufacturers are also making it far simpler to actually take the X-ray image in the first place. This vital step must be taken before any sort of fault interpretation can be applied, let alone automation of the whole procedure. The difficulty in obtaining good X-ray images for analysis, especially by those who are less familiar, or less trained, in X-ray inspection often centre around a number of key issues:
Tube kV and Power
These jargon terms refer to the penetrating power of the X-rays and the brightness of the X-ray lamp. These conditions have to be set at appropriate values such that an image of your sample, is presented to the operator with reasonable contrast. In the case of X-rays, reasonable contrast means that the X-rays should be absorbed most strongly by the more dense parts of your sample (such as BGA solder balls) and at lesser levels with those less dense areas (such as the substrate itself). If the kV is set too high for an object then the X-rays will pass straight through without any being absorbed and so saturating the detector. Conversely, if the kV is set too low then the X-rays cannot pass through the sample at all, will not get to the detector and so will not give any image at all.
Once the correct tube settings have been applied, the best images to use for analysis usually require that the raw data be enhanced. This can be looked at in the same way as adjustments made to family digital photos. As with digital photography, the PC provides the opportunity for applying a whole range of enhancements to the X-ray image. What are the best to use is subjective. However, the luxury of home photography, where you can spend a lot of time getting trying different enhancement combinations, is not available in the production environment. Therefore, the correct set of enhancements needs to be able to be applied for each view of the sample, and the required settings may need to change for each view. To help the analysis, the most typical enhancements are:
* Improving the signal-to-noise - making the final image an average taken from a series of images. The more images taken, the better the picture as the signal-to-noise ratio improves. However, more time is needed to take these extra images which may impact on sample throughput. Therefore, a compromise must be reached to suit each application.
* Applying edge enhancements -improving the sharpness of the final image. There are many possible edge enhancements and each can be applied at a variable strength factor. Again, each different view of the sample may require different enhancements at different strengths. Applying these manually each time could affect test throughput.
* Use of colour filters to replace the black and white levels. As the human eye is relatively insensitive to levels of grey, details could be missed during inspection. Using a false colour filter can make features, such as voids in BGAs, stand out very clearly.
The inclusion of a sub-micron X-ray tube in the XiDAT system means that the semiconductor market is now able to enjoy the digital imaging benefits provided by this latest advancement. It is possible to routinely achieve feature recognition as low as 600 nano-meters. Users are also able to inspect packages and devices at oblique angles of up to 45¡ from any position around the subject at a system magnification up to 1400x.
To further enhance the inspection process, manufacturers are looking at providing easier image interpretation. One approach is to have an image wizard along the lines of wizards found in all aspects of PC use today. Here the power of the PC is used in conjunction with smart, but transparent, algorithms to automatically produce improved, analytical-quality X-ray images. This provides even the novice user with a way of obtaining the data needed. All they need to do is choose which image is the best for them from a selection. There is no need to know how to change the kV, power and other enhancements - the system does it for you.
This advancement opens the door to many companies to begin to use X-ray technology. Where specifically trained individuals were once required the simpler approach allows greater access to the technology. Couple this with the reduced need for back up of analogue imagery and the cost of ownership factors become more attractive. The more cost effective and easy to use system does not prevent the more experienced user from tweaking the image yet further. In this way, more people can be trained to use the X-ray system and therefore, the opportunity is there for more testing to confirm quality without increasing, or possibly even reducing, the test expenditure.